1. Field of the Invention
Embodiments of the invention relate to a semiconductor device.
2. Description of the Related Art
Silicon (Si) has been used as a constituent material of power semiconductor devices that control high voltage and/or large current. There are several types of power semiconductor devices such as bipolar transistors, insulated gate bipolar transistors (IGBTs), metal oxide semiconductor field effect transistors (MOSFETs), etc. These devices are selectively used according to an intended purpose.
For example, bipolar transistors and IGBTs have high current density compared to MOSFETs, and can be adapted for large current but cannot be switched at high speeds. In particular, the limit of switching frequency is about several kHz for bipolar transistors and about several tens of kHz for IGBTs. On the other hand, power MOSFETs have low current density compared to bipolar transistors and IGBTs, and are difficult to adapt for large current but can be switched at high speeds up to about several MHz.
However, there has been a strong demand in the market for a power semiconductor device that achieves both large current and high speed. Thus, IGBTs and power MOSFETs have been intensively developed and improved, and the performance of power devices has substantially reached the theoretical limit determined by the material. In terms of power semiconductor devices, semiconductor materials replacing silicon have been investigated and silicon carbide (SiC) has been focused on as a semiconductor material enabling production (manufacture) of a next-generation power semiconductor device having low ON voltage, high-speed characteristics, and high-temperature characteristics (see, for example, K. Shenai, et al, “Optimum Semiconductors for High-Power Electronics”, IEEE Transactions on Electron Devices, September 1989, Vol. 36, No. 9, pages 1811-1823).
Silicon carbide is chemically a very stable semiconductor material, has a wide bandgap of 3 eV, and can be used very stably as a semiconductor even at high temperatures. Silicon carbide has a critical electric field strength that is ten times that of silicon or greater, and is expected to be a semiconductor material that can sufficiently reduce ON-resistance. These merits of silicon carbide are common to other wide bandgap semiconductors (hereinafter, wide bandgap semiconductor) having a bandgap greater than silicon, such as gallium nitride (GaN). Thus, a high-voltage semiconductor device having low resistance can be achieved by using a wide bandgap semiconductor (see, for example, B. Jayant Baliga, “Silicon Carbide Power Devices”, U.S.A, World Scientific Publishing Co., Mar. 30, 2006, page 61).
Further, as another semiconductor device realizing low resistance and high voltage, a device that raises the crystalline quality of a portion in which a channel is formed, by forming the portion (base portion) that becomes the channel (inversion layer) by epitaxial growth has been proposed (for example, refer to Japanese Laid-Open Patent Publication No. 2006-147789). In Japanese Laid-Open Patent Publication No. 2006-147789, by raising the crystalline property of the portion in which the channel is formed and reducing the channel resistance, lower resistance and higher voltage are realized.
As a semiconductor device in which a portion becoming a channel is formed by epitaxial growth, a device has been proposed in which a portion of an edge termination structure portion of the epitaxial layer that includes the portion becoming a channel is removed whereby a p-type region is provided at a step portion occurring near a boundary of the active region and the edge termination structure portion and, at the step portion, the distribution of a p-type impurity in a depth direction is gentle, the device mitigates the concentration of electric field at the step portion and prevents a reduction of the breakdown voltage (for example, refer to Japanese Laid-Open Patent Publication No. 2010-045388).